1. Field of the Invention
The present invention relates to a switching mode power supply; and, more particularly, to a switching mode power supply for reducing standby power to operate a PFC unit as well as a DC/DC unit in a burst mode in no-load or a light load.
2. Description of the Related Art
It is general that a high power density switching mode power supply is a two stage converter consisting of a PFC(Power Factor Correction) unit for taking full charge of improvement of power factor and harmonic regulation and a DC/DC unit for obtaining a desired output voltage.
In such a two stage converter, although a boost converter is often employed as the PFC unit, a buck converter is also used in order to reduce a withstanding voltage of a link voltage. Further, as the DC/DC unit, a forward converter, a converter operating by a pulse width modulation method such as a half bridge converter, a quasi-resonant flyback converter with an improved switching characteristic, and an LLC resonant converter are used.
Meanwhile, because the high power density switching mode power supply is frequently used in a home device, the study for reducing standby power has been progressed actively to meet regulation such as Energy Star.
Therefore, a recent DC/DC controller employs a burst mode type in order to minimize power consumption in no-load or a light load.
However, the PFC unit has a problem in that because a burst mode is not applied to a PFC controller operating in a CCM (Continuous Conduction Mode) or a BCM (Boundary Conduction Mode), a power switch of the PFC unit continuously operates even in the no-load or the light load, which causes serious switching loss.